Returns the NSButtonCell's key equivalent. The
key equivalent and its modifier mask are used to
simulate the click of the button in
-performKeyEquivalent:
. Returns an empty string if no key equivalent is
defined. By default NSButton hasn't key
equivalent.

Returns the modifier mask of the NSButtonCell's
key equivalent. The key equivalent and its modifier
mask are used to simulate the click of the button in
-performKeyEquivalent:
. The default mask is NSCommandKeyMask.

Sets the NSButtonCell's key equivalent to
aKeyEquivalent. The key equivalent and
its modifier mask are used to simulate the click of
the button in
-performKeyEquivalent:
. By default NSButton hasn't key equivalent.

Sets the modifier mask of the
NSButtonCell's key equivalent to
mask. The key equivalent and its
modifier mask are used to simulate
the click of the button in
-performKeyEquivalent:
. By default the mask is NSCommandKeyMask.